Vacuum degassing method for molten glass flow

ABSTRACT

When molten glass which is under an atmosphere of pressure P, is fed into a vacuum chamber capable of rendering a pressure to the molten glass to be in a range of 38 [mmHg]-(P-50) [mmHg] to perform degassing to the molten glass, a staying time of the molten glass in the vacuum chamber is in a range of 0.12-4.8 hours, whereby there is obtainable an effective degassing function to the molten glass.

This application is a continuation application of parent applicationSer. No. 09/547,669, filed Apr. 12, 2000 now U.S. Pat. No. 6,332,339.

The present invention relates to a vacuum degassing method for moltenglass flow capable of removing bubbles properly and effectively from acontinuous flow of molten glass obtained by melting glass materials.

Heretofore, it has been common to utilize a refining procedure to removebubbles generated in molten glass obtained by melting raw materials ofglass in a melting furnace, prior to forming the molten glass by aforming apparatus, in order to improve the quality of formed glassproducts.

There has been known such a method that in the refining procedure, arefining agent such as sodium sulfate (Na₂SO₄) is previously added toraw materials of glass and the molten glass obtained by melting the rawmaterials containing a refining agent is stored and maintained at apredetermined temperature for a predetermined period, during whichbubbles in the molten glass grow by the help of the refining agent, riseto the molten glass surface, and the bubbles are removed.

Further, there has been known a vacuum degassing method wherein moltenglass is introduced into a vacuum atmosphere under a reduced pressure;under such reduced pressure condition, bubbles in a continuous flow ofmolten glass grow up and rise to the molten glass surface at whichbubbles break and are removed, and the molten glass is taken out fromthe vacuum atmosphere.

In the above-mentioned vacuum degassing method, the molten glass flow isformed under a reduced pressure wherein bubbles contained in the moltenglass grow in a relatively short time and rise to the surface by usingbuoyancy of the grown-up bubbles in the molten glass, followed bybreaking the bubbles on the surface of the molten glass. In this way,the method can remove bubbles effectively from the molten glass surface.In order to remove bubbles effectively from the molten glass surface, itis necessary to provide a high rising velocity of bubbles so that thebubbles come to the molten glass surface under a reduced pressurecondition. Otherwise, the bubbles are discharged along with the moltenglass flow, with the result that a final glass product contains bubblesand is defective.

For this reason, it is considered that the pressure in the reducedpressure atmosphere for vacuum degassing should be small as possible togrow up bubbles and the rising velocity be increased whereby the effectof vacuum degassing is improved.

However, when the pressure in the reduced pressure atmosphere for vacuumdegassing is lowered, numerous new bubbles sometimes generate in themolten glass and the bubbles rise to the molten glass surface to form afloating foam layer without breaking. A part of the foam layer may bedischarged along with the molten glass flow to result a defect in theglass product. When a foam layer grows, the temperature of the uppersurface of the molten glass decreases. The foam layer tends to hardlybreak whereby the foam layer will further develop. As a result, theinside of the vacuum degassing apparatus is filled with non-breakingbubbles. The foam layer fully filling the apparatus may be in contactwith impurities on the ceiling of the apparatus; thus, it brings theimpurities in the final glass product. Consequently, excessivelylowering the pressure in the atmosphere for vacuum degassing is notpreferred for an effective treatment for vacuum degassing.

Further, the rising velocity of the bubbles in molten glass isdetermined by the viscosity of the molten glass as well as the size ofthe bubble. Accordingly, it is considered that the lowering of theviscosity of the molten glass, or the raising of the temperature of themolten glass can raise bubbles to the surface effectively. However, whenthe temperature of the molten glass is excessively raised, there causesan active reaction with the material of flow path, such as refractorybricks, with which the molten glass contacts. It may lead to occurrenceof new bubbles and dissolution of a part of material of the flow pathinto the molten glass, thus resulting in deterioration of the quality ofglass products. Further, when the temperature of the molten glass israised, the strength of the material of the flow path is decreased,whereby the service life of the flow path is shortened and an extraequipment such as a heater for maintaining the high temperature of themolten glass is required. As a result, in order to conduct a proper andeffective vacuum degassing treatment, it is difficult to lowerexcessively the pressure for vacuum degassing and also to raiseexcessively a temperature of the molten glass.

In the vacuum degassing method where several restrictions are imposed,the following conditions for effective degassing has been reported(SCIENCE AND TECHNOLOGY OF NEW GLASSES, Oct. 16-17, 1991, pages 75-76).

In a vacuum degassing apparatus 40 for carrying out a vacuum degassingmethod for a molten glass flow as shown in FIG. 4, the number of bubbles(bubble density) in molten glass decreases to about 1/1,000, when moltenglass at 1,320° C. is passed in the apparatus at a flow rate of 6[ton/day] wherein a pressure in a vacuum degassing vessel 42 is 0.18atom (136.8 mmHg) and a staying time of the molten glass in the vacuumdegassing vessel 42 under such a reduced pressure atmosphere is 50minutes.

Namely, the above-mentioned vacuum degassing treatment is conducted in abench scale type vacuum degassing apparatus 40 in the following way. Themolten glass obtained by melting raw materials of glass is introducedfrom an upstream pit 47 into the vacuum degassing vessel 42 under areduced pressure via an uprising pipe 44 by a vacuum pump (not shown),whereby a molten glass flow is formed in a substantially horizontaldirection. Then, the molten glass is passed in the vacuum degassingvessel 42 under a reduced pressure to remove the bubbles in the moltenglass, and then, the molten glass is fed via a downfalling pipe 46 to adownstream pit 48 where the temperature of the molten glass ismaintained to have the viscosity of 1,000 poises.

The molten glass is sampled at the inlet of the uprising pipe 44 and theoutlet of the downfalling pipe 46 to check the bubble density containedin each sample of the molten glass. As a result, the bubble densitycontained in the molten glass in the upstream pit 47 prior to a vacuumdegassing is 150 [number/kg] and the bubble density contained in themolten glass in the downstream pit 48 is 0.1 [number/kg]. Thus, it isrecognized that the number of the bubbles decreases to about 1/1,000. Itis also reported that a foam layer is not formed on the molten glasssurface, although the pressure in the vacuum degassing vessel 42 is setto be low level as 0.18 atom.

The above-mentioned report discloses the vacuum degassing method whereinan effective vacuum degassing is attained when a pressure in the vacuumdegassing vessel 42 is 0.18 atom (136.8 mmHg) and a staying time in thevacuum degassing vessel 42 is 50 minutes. However, it does not disclosevarious condition requirements for the vacuum degassing in order toobtain effectively superior results of vacuum degassing.

In particular, a vacuum degassing treatment should be carried out withina relatively short time under a reduced pressure atmosphere.Accordingly, under such conditions where the pressure in the reducedatmosphere can not be lowered excessively and the temperature of themolten glass can not be excessively high, as mentioned above, it isimportant to determine a staying time of the molten glass flow under thereduced pressure atmosphere.

The longer the staying time of the molten glass flowing in the vacuumdegassing vessel 42, the uprising pipe 44 and the downfalling pipe 46,the lower the bubble density of the molten glass after vacuum degassingtreatment.

In order to elongate the staying time of the molten glass under areduced pressure atmosphere, it is considered to extend the length offlow path of the molten glass in a flow direction. However, this causespractical problems such as a remarkable increase in cost of theequipment due to the reasons as follow. Since an insulator forinsulating a high temperature of the molten glass and a housing as acasing to maintain a reduced pressure, which surrounds the insulator andmaterials for the flow path, are provided at an outer periphery of theflow path for passing the molten glass of high temperature, theinsulator and the housing must be extended according to the extension ofthe flow path. Further, a heavy structural unit comprising the materialsfor the flow path, the insulator and the housing must be movable so thatthe height of the unit can be adjusted depending on a pressure in thevacuum degassing vessel 42. This creates a large-sized movableequipment, hence, cost of the equipment will increase.

It is considered that the staying time can be extended by lowering theflow velocity of the molten glass. However, in order to lower the flowvelocity, it is necessary to increase the viscosity by decreasing thetemperature of the molten glass. In this case, it is difficult to raisethe bubbles in the molten glass having a high viscosity to the moltenglass surface.

On the other hand, when the staying time of the molten glass under areduced pressure atmosphere is shortened excessively, sufficientdegassing of the bubbles in the molten glass can not be achieved.Namely, a sufficient time for growing the bubbles in the molten glassunder a reduced pressure atmosphere to raise them to the molten glasssurface to thereby remove the bubbles by breaking can not be obtained,with the result that the molten glass with the bubbles may be dischargedbefore the bubbles reach the molten glass surface. Although it ispossible to lower the viscosity of the molten glass, i.e., to elevatethe temperature of the molten glass in order to increase the risingvelocity of bubbles in the molten glass, the temperature of the moltenglass can not be increased because of the problems of a reduction instrength of the materials used for the flow path for the molten glassand the occurrence of new bubbles caused by the reaction of thesematerials with the molten glass.

It is an object of the present invention to provide a vacuum degassingmethod for molten glass flow, which is capable of obtaining effectivelyand certainly molten glass without containing bubbles by specifying arange of staying time of the molten glass in a case of conducting adegassing treatment to a continuous flow of molten glass under a reducedpressure atmosphere.

Further, the present invention aims at determining a proper range ofvacuum degassing conditions for the molten glass under a reducedpressure atmosphere in the above-mentioned vacuum degassing method sothat molten glass without containing bubbles can further be effectivelyand certainly obtained.

The inventors of this application have made extensive studies on vacuumdegassing methods for molten glass flow to achieve the above-mentionedobjects, and have found that it is necessary to make bubbles grown inmolten glass to raise them to the molten glass surfaces where thebreaking of the bubbles takes place, whereby the bubbles in the moltenglass can effectively and certainly be removed. Thus, the presentinvention has been accomplished by satisfying the below-mentionedconditions:

1. The molten glass is continuously passed.

2. A condition that new bubbles are not generated is provided.

3. The diameter of bubbles is increased in a prescribed time so as tohave a sufficient buoyancy.

4. The rising velocity of bubbles is provided to the bubbles so as to beagainst the molten glass flow.

5. A sufficient amount of gases to be diffused into the bubbles isassured so that the bubbles reaching the molten glass surface can bebroken.

In accordance with the present invention, there is provided a vacuumdegassing method for molten glass which comprises feeding, under anatmosphere of pressure P [mmHg], molten glass into a vacuum chambercapable of rendering a pressure to the molten glass to be in a range of38 [mmHg]-(P-50) [mmHg] to perform degassing to the molten glass, anddischarging the molten glass after having been degassed from the vacuumchamber at a flow rate of Q [ton/hr] under the atmosphere of pressure P[mmHg] wherein a staying time of the molten glass in the vacuum chamberis in a range of 0.12-4.8 hours, which is obtained by dividing a weightW [ton] of the molten glass flowing in the vacuum chamber by a flow rateQ [ton/hr] of the molten glass. In this case, the staying time in thevacuum chamber is preferably not less than 0.12 hour but not more than0.8 hour.

Further, the vacuum chamber preferably includes a vacuum degassingvessel in which the molten glass is passed in a substantially horizontalstate and is degassed, and a depth H [m] of the molten glass in thevacuum degassing vessel and a weight W [ton] of the molten glass satisfythe below-mentioned Formula (1):

0.010 m/ton<H/W<1.5 m/ton.  (1)

Further a surface area S₁ [m²] of the molten glass surface in the vacuumdegassing vessel and a flow rate Q [ton/hr] of a molten glass flowpreferably satisfy the below-mentioned formula (2):

0.24 m²·hr/ton<S ₁ /Q<12 m²·hr/ton  (2)

Further, the vacuum chamber preferably includes a downfalling pipeconnected to the vacuum degassing vessel to discharge the molten glasstherethrough, and a surface area S₂ [m²] of flow path of the downfallingpipe at the portion where the downfalling pipe is connected to thevacuum degassing vessel and a flow rate Q [ton/hr] of the molten glasssatisfy the below-mentioned Formula (3):

 0.008 m²·hr/ton<S ₂ /Q<0.96 m²·hr/ton  (3)

In drawings:

FIG. 1(a) is a diagrammatical cross-sectional view for explaining animportant portion of a vacuum degassing apparatus for carrying out thevacuum degassing method for molten glass flow according to the presentinvention;

FIG. 1(b) is a diagrammatical cross-sectional view taken along a lineB-B′ in FIG. 1(a);

FIG. 1(c) is a diagrammatical cross-sectional view taken along a lineC-C′ in FIG. 1(a);

FIG. 2 is a diagrammatical cross-sectional view of a vacuum degassingapparatus for carrying out the vacuum degassing method for molten glassflow according to an Example of the present invention;

FIG. 3 is a diagrammatical cross-sectional view of a vacuum degassingapparatus for carrying out the vacuum degassing method for molten glassflow according to another Example of the present invention; and

FIG. 4 is a diagrammatical cross-sectional view of a vacuum degassingapparatus for carrying out a conventional vacuum degassing method formolten glass flow.

Preferred embodiments of the vacuum degassing method for molten glassflow of the present invention will be described with reference to thedrawings.

As described above, the present invention concerns a vacuum degassingmethod for molten glass flow to conduct degassing in a vacuum chamberwherein a range of staying time of the molten glass flowing continuouslyin the vacuum chamber is specified whereby molten glass free frombubbles can effectively and certainly be obtained.

Description will be made as to such vacuum degassing method withreference to FIGS. 1(a), 1(b) and 1(c).

FIGS. 1(a)-1(c) are diagrams for explaining important portions of avacuum degassing apparatus for carrying out the vacuum degassing methodfor molten glass flow according to the present invention. The vacuumdegassing method of the present invention comprises mainly a vacuumdegassing step of removing bubbles in molten glass flowing in asubstantially horizontal state under a reduced pressure atmosphere, anintroducing step to introduce molten glass to be degassed to the vacuumdegassing step and a discharge step to discharge the molten glassdegassed in the vacuum degassing step.

In FIG. 1(a), the introducing step to introduce molten glass from amelting vessel 10 in which the molten glass obtained by melting rawmaterials of glass is stored under an atmosphere of pressure P[mmHg]into a vacuum degassing vessel 12 in which the vacuum degassing step iscarried out, is conducted in an uprising pipe 14, during which a moltenglass flow is formed. The vacuum degassing step to raise bubblesremaining in the molten glass flowing in a substantially horizontaldirection to the molten glass surface under a reduced pressureatmosphere and to remove by breaking them at the molten glass surface,is conducted mainly in the vacuum degassing vessel 12. The dischargingstep to discharge the molten glass degassed in the vacuum degassingvessel 12 from a downstream pit 18 through the vacuum degassing vessel12 is conducted in a downfalling pipe 16. The major portions of theuprising pipe 14 and the downfalling pipe 16 as well as the vacuumdegassing vessel 12 for degassing are covered with a vacuum housing (notshown) connected to a vacuum pump, and evacuation of the vacuumdegassing vessel is conducted through openings 12 a, 12 b formed in theceiling of the vacuum degassing vessel 12 so as to maintain the reducedpressure to be constant.

A typical value of pressure P in this case is 760 [mmHg].

As described before, in such vacuum degassing method wherein bubbles inthe molten glass flowing in the vacuum degassing vessel 12 are madegrown and are raised in the molten glass to break them on the moltenglass surface, a staying time of the molten glass in the vacuumdegassing vessel 12, i.e., a time during which the molten glass ispassed through the vacuum degassing vessel 12 can not be shortenedexcessively. It is also difficult to shorten excessively a time in whichthe molten glass rises to pass through the uprising pipe 14 even in theintroducing step wherein the molten glass stored in the melting vessel10 under an atmosphere of pressure P [mmHg] is sucked and raised intothe vacuum degassing vessel 12 in a reduced pressure condition. It isbecause the pressure in a lower portion of the uprising pipe 14 is highdue to the own weight of the molten glass, and the pressure in an upperportion of the uprising pipe 14 becomes gradually small toward themolten glass surface in the vacuum degassing vessel 12. Accordingly,when the molten glass rises in the uprising pipe 14, the pressure givento the molten glass is lower than the pressure P [mmHg] which isoperated when the molten glass is obtained by melting raw materials. Asa result, the bubbles in the molten glass grow while they pass throughthe uprising pipe 14. Further, new bubbles formed by gases dissolved inthe molten glass grow while they rise in the uprising pipe 14.

Further, it is difficult to shorten excessively a time in which themolten glass passes through the downfalling pipe 16. The reason is asfollows. As the molten glass descends in the downfalling pipe 16, thepressure to the molten glass gradually increases due to the own weightof the molten glass from a reduced pressure level in the vacuumdegassing vessel 12. The pressure is finally restored to have theabove-mentioned pressure P [mmHg]. However, the bubbles which are notremoved even by a reduced pressure in the vacuum degassing vessel 12dissolve to be gas components in the molten glass due to a pressurewhich increases as the molten glass descends in the downfalling pipe 16.

For this, the present invention provides a vacuum chamber which rendersa pressure applied to the molten glass to be in a range of 38[mmHg]-(P-50) [mmHg] with respect to a pressure P [mmHg], and includes atime in which the molten glass passes not only through the vacuumdegassing vessel 12 but also parts of the uprising pipe 14 and thedownfalling pipe 16. The reason why the pressure in the vacuum chamberis to be 38 [mmHg] or more is that an unexpected discharge (reboil) ofdissolved gases in the vacuum chamber can be suppressed, as describedbefore. The vacuum chamber defined as described above corresponds to aroughly hatched portion in FIG. 1.

In order to feed the molten glass continuously in the vacuum chamber, itis necessary to design a flow path for the vacuum chamber so as toreduce the frictional resistance between an inner surface of the flowpath of the vacuum chamber and the molten glass flow and to reducesufficiently the pressure loss of fluid. In order to reduce sufficientlythe pressure loss of fluid, designing of the shape and thecross-sectional surface area of the flow path of the vacuum chamber hasbeen property conducted. However, since it is desirable that bubblesgenerated in the molten glass are inflated in a shorter time while themolten glass is passed continuously, whereby the bubbles are raised tothe molten glass surface on which the breakage of the bubbles is caused,it is considered to lower the viscosity of the molten glass, i.e., todetermine the temperature of the molten glass to be high. However, asdescribed above, when the temperature of the molten glass is elevated,new bubbles are generated by the reaction of the materials used for theflow path of the vacuum chamber with the molten glass, or the materialsdissolve into the molten glass to form cord, with the result that thequality of formed products can not be maintained. Further, the reactionof these materials with the molten glass accelerates erosion of thematerials and the service life of the flow path for the vacuum chamberis shortened.

The rate of erosion of the flow path of the vacuum chamber caused by themolten glass flow is in proportion to t/η, i.e., the ratio of a time tto the viscosity η of the molten glass wherein t represents a time inwhich the molten glass is passed through the flow path and η representsthe viscosity of the molten glass. The rising distance of bubbles whenthe bubbles rise to the molten glass surface is in proportion to thesquare of t/η, i.e., the ratio obtained by dividing a time t in whichthe molten glass is passed through the flow path by the viscosity η ofthe molten glass. Accordingly, it is desirable to determine theviscosity of the molten glass to be lower within a permissible range onthe rate of erosion, in order that a sufficient rising distance of thebubbles can be maintained.

A preferred range of the viscosity of the molten glass is 500-5,000poises. Further, in order to raise bubbles in the molten glass havingthe viscosity of such range to the molten glass surface, the bubblesshould have a diameter of 10-30 mm. In this case, when the diameter ofthe bubbles exceeds 30 mm, the bubbles reaching the surface do not breakand a foam layer remains on the surface. This reduces heat transferefficiency in the vacuum degassing vessel 12 and the temperature of themolten glass itself is reduced whereby the vacuum degassing effectdecreases.

An analysis of gas has revealed that bubbles rise to the molten glasssurface in the vacuum degassing vessel 12 to emit CO₂ and H₂O. In thiscase, there is found through direct observation of the inside of thevacuum degassing vessel 12 that an unexpected discharge (reboil) ofdissolved gasses such as CO₂, H₂O and so on in the molten glass easilytakes place under a certain pressure (limit pressure) or lower. Suchreboil occurs at a limit pressure of 0.05 atm in the molten glass havinga viscosity of, for example, 500-5,000 poises, and accordingly, it ispreferable to conduct the degassing under an atmosphere of such pressureor higher.

Further, in order that the diameter of the bubbles is increased so thatthe bubbles have a sufficient buoyancy in a time in which the moltenglass is passed through the vacuum chamber, it is necessary to diffuseor introduce gas components existing in a dissolving state in the moltenglass into small bubbles, e.g., bubbles having a diameter of 0.05-3 mm,containing in the molten glass in the melting vessel 10 under a reducedpressure atmosphere in the vacuum degassing vessel 12 so as not to causethe generation of the reboil. The reasons are as follows. It isdifficult to grow the bubbles by introducing gas components into smallbubbles existing in the molten glass because the gas components have ahigh partial pressure under an atmosphere wherein the molten glass isobtained in the melting vessel 10, i.e., under the atmosphere ofpressure P. Further, an attempt of bubbling to the molten glass so as toaccelerate the introduction of the gas components into the bubbles byincreasing gas components in the molten glass can not providepractically a sufficient effect.

In consideration of the above-mentioned, there is employed suchtechnique that a molten glass flow is produced by passing the moltenglass; small bubbles are made growing in a time in which the moltenglass stays in the vacuum chamber; the bubbles growing in a reducedpressure atmosphere are upraised to the molten glass surface of thevacuum degassing vessel 12 to break the bubbles whereby the bubbles areremoved, and bubbles which can not be sucked and removed byvacuum-degassing are made dissolved into the molten glass in thedownfalling pipe 16 to thereby eliminate all the bubbles in the moltenglass. In this case, according to the present invention, a staying timeof the molten glass in the vacuum chamber, which is obtained by dividinga weight W [ton] of the molten glass flowing in the vacuum chamber by aflow rate Q [ton/hr] of the molten glass, is in a range of 0.12-4.8hours, more preferably, in a range of 0.12-0.8 hour.

Here, the weight W [ton] of the molten glass flowing in the vacuumchamber implies the total weight of the molten glass in the vacuumchamber (in a portion indicative of a roughly hatched portion in FIG.1(a)).

When the staying time is shorter than 0.12 hour, the bubble density ofthe molten glass can not be within a permissible range for good finalproducts even though the viscosity of the molten glass is 500-5,000poises and the pressure to the molten glass is 0.05 atmosphericpressure, i.e., 76 [mmHg] or higher. On the other hand, when the stayingtime is longer than 4.8 hours, the elongation of the vacuum chamber in adirection of flowing the molten glass is required, which invites apractical problems of increasing cost for the equipment.

A staying time of less than 0.8 hour provides preferred effects ofremoving efficiently the bubbles, and reducing the volatilization ofvolatile components from the molten glass surface.

Further, it is preferable that a depth H [m] of the molten glass in thevacuum degassing vessel 12 and a weight W [ton] of the molten glassflowing in the vacuum chamber satisfy the below-mentioned formula:

0.010 m/ton<H/W<1.5 m/ton

H/W is preferably 0.012 m/ton or higher, more preferably 0.015 m/ton orhigher. Further, H/W is preferably 1.2 m/ton or lower, more preferably,0.9 m/ton or lower.

The reason that the ratio of the depth H [m] of the molten glass in thevacuum degassing vessel 12 to the weight W [ton] of the molten glass bewithin the above-mentioned range, is as follows.

If the depth H of the molten glass in the vacuum degassing vessel 12 is0.010×W or lower, there is an increase of pressure loss due to africtional resistance of the molten glass flow and it is impossible topass the molten glass at a predetermined flow rate. On the other hand,if the depth is 1.5×W or higher, the bubbles of molten glass existing ator around the bottom of the vacuum degassing vessel 12 can not float tothe molten glass surface while the molten glass stays in the vacuumdegassing vessel 12. Further, when the depth of the molten glass in thevacuum degassing vessel exceeds 1.5×W as an upper limit of theabove-mentioned range, the pressure to the molten glass staying at oraround the bottom of the vacuum degassing vessel 12 is high and thegrowth of the bubbles of molten glass in that region is not acceleratedwhereby the bubbles can not rise to the molten glass surface and thereis a case that the bubbles flow out from the vacuum degassing vessel.

A predetermined degassing effect is obtainable even by introducing themolten glass into the vacuum degassing vessel 12 to the full extent ofthe upper limit of the above-mentioned degassing-permissible range.However, it is preferred that the depth of the molten glass is abouthalf as much as the height of the vacuum degassing vessel. For example,when the height of the vacuum degassing vessel 12 is 0.2 m-0.6 m, thedepth of the molten glass should be in a range of 0.1 m-0.3 m.

In FIG. 1(a), the inside of the vacuum degassing vessel 12 has a shapeof rectangular prism wherein the shape of flow path in cross section isrectangular and the depth H [m] of the molten glass in the vacuumdegassing vessel 12 is constant. However, the present invention is notlimited to the case that the inside of the vacuum degassing vessel is ina rectangular prism but the present invention is applicable to a casethat the bottom surface of the vacuum degassing vessel is gradually orstepwisely raised or lowered from the upstream portion to the downstreamportion of the vacuum degassing vessel while the surface of the ceilingof the vessel is maintained at a certain level. In this case, a depth inaverage of the molten glass means the depth H [m] of the molten glass.

Further, the inside of the vacuum degassing vessel 12 may be in acylindrical column shape wherein the shape of the flow path in crosssection is circular. In this case, the depth H [m] of the molten glassmeans the depth in the deepest portion among depths varying along awidth direction. In this case, the bottom surface of the vacuumdegassing vessel 12 may be gradually or stepwisely raised or loweredfrom the upstream portion to the downstream portion of the vessel forflowing the molten glass. In determination of the depth H [m] of themolten glass, it is obtained by simply averaging depths of the moltenglass flow.

As described above, it is necessary to assure the introduction of thedissolved gas components into bubbles as much as possible so that thebubbles in the molten glass rise in the molten glass to cause thebreakage of them. In this case, the bubbles reaching the molten glasssurface to form a foam layer unless they are broken. The foam layer hasa heat insulating effect, and prevents the breakage of bubbles inassociation with a reduced temperature at the molten glass surface. Whenthe foam layer grows, the foam layer may flood over the vacuum degassingvessel 12, or may be discharged from the vacuum degassing vessel 12along with the molten glass flow.

From this standpoint, the breakage of the bubbles is essential. However,the breakage of bubbles depends on a temperature of the molten glasssurface and a rate of introduction of gases into the bubbles as well asthe surface tension of each bubble forming the foam layer and theviscosity of the molten glass forming each bubble. Accordingly, when aformulation for the molten glass and a temperature for a vacuumdegassing treatment for the molten glass are determined, a relation ofan air-contacting surface area of the molten glass, which is necessaryfor the breakage of the bubbles, to a flow rate of the molten glass hasto be determined to be a predetermined range.

Namely, in the process of raising the bubbles in the molten glass in thevacuum degassing vessel 12 to the molten glass surface where thebreakage of the bubbles is caused while the molten glass is passedthrough the inside of the vessel 12, and gas components contained in thebubbles are discharged to an upper space 12 s under a reduced pressurecondition, it is preferable, in the present invention, for causing thebreakage of the bubbles that the surface area of the molten glass S₁[m²] (the surface area in a roughly hatched portion shown in FIG. 1(b))which contacts the upper space 12 s under a reduced pressure conditionand the flow rate Q [ton/hr] of the molten glass satisfy the followingformula:

0.24 m²·hr/ton<S ₁ /Q<12 m²·hr/ton

More preferably, they should satisfy the following formula:

 0.5 m²·hr/ton<S ₁ /Q<10 m²·hr/ton

The reason why the above-mentioned formulas are established is asfollows. If the surface area S₁ [m²] of the molten glass surface in thevacuum degassing vessel 12 is 0.24×Q or lower, a large number of bubblesrising to the molten glass surface stay on the surface to produce a foamlayer which remains unbroken in the vacuum degassing vessel 12, wherebythe degassing treatment can not properly be conducted. On the otherhand, if the surface area S₁ [m²] is 12×Q or higher, the molten glass inthe vacuum degassing vessel 12 has a shallow depth whereby the moltenglass can not be passed at a predetermined flow rate due to a frictionalresistance caused by the molten glass flow.

In FIG. 1(b), the molten glass surface of the molten glass whichcontacts the upper space 12 s under a reduced pressure condition has arectangular shape. However, in the present invention, the shape of themolten glass surface is not limited to this but it may have such a shapethat an inner width of the vacuum degassing vessel 12 is gradually orstepwisely narrowed or broadened from the upstream portion to thedownstream portion of the vessel 12.

Further, a rate of rising of the bubbles in the molten glass flow whichrise as they grow, is related by the diameter of the bubbles and Stoke'sformula. When the viscosity of the molten glass is determined, a timerequired for the rising of bubbles to the molten glass surface isdetermined depending on the size of the bubbles. For example, when theviscosity of the molten glass used is 500-5,000 poises and assuming thatfor bubbles, it takes 60 min for floating a distance of 100 cm, thesmallest diameter of the bubbles should be 10 mm or more in a case of500 poises, and the smallest diameter be 30 mm or more in a case of5,000 poises. Namely, the bubbles having a diameter of 30 mm or more cancertainly be degassed and removed in a time of 60 min. In this case, therate of rising of 0.25 cm/sec or more is obtainable.

Accordingly, in order to assure the rising of the bubbles against themolten glass flow, it is necessary to determine the flow rate of themolten glass to be a rate lower than 0.25 cm/sec (for example, when themolten glass is passed at a flow rate of 500 ton/day, the surface areaof the flow path in cross section in the vacuum degassing vessel 12 is9,200 cm² or more and the length of flow path in the vacuum degassingvessel 12 is about 1 m).

In this case, as shown in FIG. 1(a), the downfalling pipe 16 is providedto descend the molten glass wherein a falling flow is formed at or nearan outlet port of the vacuum degassing vessel 12 connected with thedownfalling pipe 16. When a rate of rising of the bubbles in the moltenglass is lower than a rate of falling of the falling flow, bubblesgrowing in the molten glass are entrained by the falling flow withoutrising to the molten glass surface at or near the outlet port connectedwith the downfalling pipe 16, with the result that it is a danger ofdischarging the molten glass containing therein the bubbles.

Accordingly, in the present invention, the surface area S₂ [m²] of thesurface in cross-section of the flow path of the downfalling pipe 16(the surface area in a roughly hatched portion in FIG. 1(c)) which isconnected to the vacuum degassing vessel 12 and the flow rate Q [ton/hr]of the molten glass preferably satisfy the following formula. Namely,either only the downfalling pipe 16 or both of the downfalling pipe 16and the uprising pipe 14 preferably satisfy the following formula:

0.008 m²·hr/ton<S ₂ /Q<0.96 m²·hr/ton

More preferably, they should satisfy:

0.01 m²·hr/ton<S ₂ /Q<0.96 m²·hr/ton,

in particular,

0.01 m²·hr/ton<S ₂ /Q<0.1 m²·hr/ton.

The reason why they should satisfy the above-mentioned formulas is asfollows.

When the surface area S₂ [m²] in cross section of the flow path of thedownfalling pipe 16 is 0.008×Q or lower, a downward vector in the flowvelocity of the molten glass flow at or near the outlet port connectedwith the downfalling pipe 16 is increased whereby bubbles are entrainedby the molten glass flow in the downfalling pipe 16 against a floatingaction. On the other hand, when the surface area S₂ [m²] in crosssection of the flow path is 0.96×Q or more, the diameter of thedownfalling pipe 16 is increased, which will increase the weight and thecost for the equipment.

In the embodiment shown in FIG. 1(c), the shape of the flow path incross section is rectangular. However, the present invention is notlimited to have such shape but a circular shape may be used for example.

The present invention concerns a vacuum degassing method for degassingmolten glass under an atmosphere of pressure P [mmHg]. However, theatmosphere of pressure P [mmHg] is not always necessary as an atmosphereof atmospheric pressure. For example, it may be an atmosphere under anoptional pressure which is employable in a case that molten glass isproduced in a closed melting vessel which is isolated from atmosphericpressure. Further, the molten glass under an atmosphere of pressure P[mmHg] may not have a free surface thereof.

Now, the vacuum degassing method for molten glass according to thepresent invention will be described in detail with reference toExamples. However, it should be understood that the present invention isby no means restricted by such specific Examples.

In Examples, the degassing of molten glass flow was conducted undervarious conditions as described below to examine the number of bubblescontained in the molten glass, i.e., the bubble densities, before andafter the degassing treatment. Further, a vacuum degassing apparatus 20as shown in FIG. 2 was used to conduct the degassing treatment of moltenglass flow.

The vacuum degassing apparatus 20 shown in FIG. 2 was basically anapparatus for producing a molten glass flow along arrow marks in FIG. 2by utilizing a principle of siphon caused by a difference of surfacelevels of molten glass in an upstream pit 21 and a downstream pit 28 toeffect degassing to molten glass in a vacuum degassing vessel 22. Theapparatus 20 was provided with a vacuum housing 23, the vacuum degassingvessel 22, an uprising pipe 24 and a downfalling pipe 26 which areconstituted in one piece. Molten glass G was filled in the upstream pit21 and the downstream pit 28, and the positions in height of the vacuumhousing 23, the vacuum degassing vessel 22, the uprising pipe 24 and thedownfalling pipe 26 were adjacent properly depending on a pressure inthe vacuum degassing vessel 22.

There was used the vacuum housing 23 which is made of a metallic casinghaving a substantially gate shape to maintain a hermetic property in thevacuum degassing vessel 22, the uprising pipe 24 and the downfallingpipe 26 and which is so constructed as to house the vacuum degassingvessel 22 and major portions of the uprising pipe 24 and the downfallingpipe 26; provides a reduced pressure condition in the inside of them bysucking air by means of a vacuum pump (not shown) provided at anexterior side and to maintain a reduced condition of a predeterminedpressure through opening 22 a and 22 b formed in the vacuum degassingvessel 22 housed therein. Further, a heat insulating material 27 toblock heat was arranged in a space surrounded by the vacuum degassingvessel 22, the uprising pipe 24, the downfalling pipe 26 and the vacuumhousing 23.

A vacuum chamber suffering a pressure higher than 38 [mmHg] (0.05atmospheric pressure) and lower than (P₀-50) [mmHg] with respect toatmospheric pressure P₀ [mmHg], as a result of evacuating the vacuumhousing 23, was formed in the vacuum degassing vessel 22, the uprisingpipe 24 and the downfalling pipe 26. Specifically, the vacuum chamberwas formed in the vessel 22, the uprising pipe 24 and the downfallingpipe 26 to extend in a portion which was higher in height level than aheight level Z₁ with respect to the surface of the molten glass G in themelting vessel 25. Accordingly, the weight W [ton] of the molten glassflowing in the vacuum chamber corresponded to the entire weight of themolten glass contained in the uprising pipe 24, the vacuum degassingvessel 22 and the downfalling pipe 26 in an area extending from theheight level Z₁ with respect to the molten glass G in the melting vessel25 to the level of the surface of the molten glass G in the vacuumdegassing vessel 22 (i.e., the molten glass existing in a roughlyhatched portion in FIG. 2).

In this Example, the cross-sectional shape of flow paths in the vacuumdegassing vessel 22, the uprising pipe 24 and the downfalling pipe 26may be circular or rectangular. The depth H of the molten glass flow wasconstant in the flowing direction of the molten glass. Further, thewidth of the molten glass flow was made constant so that the shape ofthe surface of the molten glass flow which contacts an upper space 22 sunder a reduced pressure condition was rectangular.

In Examples 1-6 shown in Table 1, molten glass of kinds as shown bycharacters A-E, with which compositions are shown by weight % in Table2, were used, and the above-mentioned vacuum degassing apparatus 20 wasused to conduct degassing treatment under conditions of molten glasstemperature [°C.] in Table 1.

In all Examples 1-6, sampling of the molten glass G in the upstream pit21 and the downstream pit 28 was conducted after normal operations forthe degassing treatments was started, and examination was made by anedge light method whether or not bubble densities were within apermissible range. In this case, the permissible range of bubble densitywas 1 [number/kg] or lower.

TABLE 1 Reference Example 1 2 3 4 5 6 Example Kind of glass A A B C D E— Pressure in vacuum degassing vessel 53 68 190 84 152 91 — Temperatureof molten glass 1350 1340 1320 1400 1350 1300 — Flow rate Q of moltenglass 0.050 0.354 0.104 0.833 0.917 1.458 16.667 Weight W of moltenglass 0.200 0.346 0.210 1.200 1.100 1.200 13.800 Depth H of molten glass0.150 0.150 0.175 0.200 0.175 0.200 0.250 Surface area S₁ of moltenglass surface 0.480 0.720 0.630 2.400 2.200 2.400 16.200 Cross-sectionalsurface area S₂ 0.0044 0.0120 0.0013 0.0450 0.0310 0.0450 0.5670 W/Q4.000 0.977 2.019 1.441 1.200 0.823 0.828 H/W 0.750 0.433 0.833 0.1670.159 0.167 0.018 S₁/Q 9.600 2.034 6.058 2.881 2.399 1.646 0.972 S₂/Q0.088 0.034 0.013 0.054 0.034 0.031 0.034 Bubble density beforedegassing treatment 500 220 7000 3000 200 300 — Bubble density afterdegassing treatment 0.1 0.2 0.5 0.6 0.5 0.2 —

TABLE 2 Kind of glass SiO₂ Al₂O₃ B₂O₃ ZrO₂ Na₂O K₂O MgO CaO SrO BaO CeO₂Sb₂O₃ SO₃ Cl A 60.0 2.0 — 2.4 8.0 7.0 0.5 2.0 8.5 9.0 0.3 0.3 — — B 75.03.5 — — 15.0 0.3 6.0 — — — — — 0.2 — C 58.0 11.0 6.0 — — — 2.0 3.0 6.715.0 — — — 0.3 D 71.5 2.0 — — 13.0 0.3 4.0 9.0 — — — — 0.2 — E 57.9 7.0— 3.0 4.0 6.0 2.0 5.0 7.0 8.0 — — 0.1 —

In all the cases of Examples 1-6, the number of bubbles per unit weightbefore the vacuum degassing treatment was small and was within thepermissible range as shown in Table 1, and there was no possibility ofcausing reduction in the quality of glass products.

From the above-mentioned Examples, it was revealed that the bubbledensities were all within the permissible range and the degassing effectcould effectively and certainly be obtained by providing the conditionsas follows. Namely, molten glass was fed into a vacuum chamber whichrendered a pressure to the molten glass to be higher than 38 [mmHg] andlower than (P₀-50) [mmHg], in which degassing of the molten glass wascarried out, and the molten glass after the degassing was discharged ata flow rate of Q [ton/hr] under an atmosphere of pressure P₀ [mmHg]wherein a staying time of the molten glass in the vacuum chamber whichis obtained by dividing the weight W [ton] of the molten glass flowingin the vacuum chamber by the flow rate Q [ton/hr] of the molten glasswas in a range of 0.12-4.8 hours.

In this case, it was also revealed that the depth of the molten glassand the surface area of the molten glass surface in the vacuum degassingvessel and the surface area in cross section of the flow path in eitherthe uprising pipe or the downfalling pipe were preferably within apredetermined ranges.

As in Reference Example, when a flow rate Q of the molten glass is largeas 16.667 [ton/hr] (i.e., about 400 [ton/day]), it is desirable that theweight W of the molten glass in the vacuum chamber is 13.8 [ton]; W/Q is0.828 [hr], and values for other conditions be as shown in ReferenceExample in Table 1.

As described above, detailed explanation has been made as to thedegassing method for molten glass of the present invention. However, thepresent invention is not limited to the above-mentioned Examples. Forexample, as shown in FIG. 3, a vacuum degassing apparatus 30 can beconstructed so that a melting vessel 35, an introducing pipe 34, avacuum degassing vessel 32, a discharge pipe 36 and a downstream pit 38are formed in one piece; a screw pump 31 is provided in the introducingpipe 34 to control a flow rate of the molten glass G; a screw pump 39 isprovided in the discharge pipe 36 to accelerate discharging of themolten glass G, and the surface level of the molten glass is always madecoincide with the level of the surface of the molten glass G in themelting vessel 35.

A vacuum chamber capable of providing a pressure which is higher than 38[mmHg] and lower than (P₀-50) [mmHg] with respect to atmosphericpressure P₀ [mmHg] as a result of reducing pressure in the vacuumhousing 33 is formed in a portion in the uprising pipe 34 a, the vacuumdegassing vessel 32 and the downfalling pipe 36 a (i.e., a portionindicated by a roughly hatched portion in FIG. 3), wherein the portionis formed in an area from the front surface of the molten glass G in themelting vessel 35 to a level lower by Z₂ than the surface of the moltenglass G. Thus, the formation of the vacuum chamber providing a pressureof (P₀-50) [mmHg] or lower in a portion which is lower in height levelthan the surface of the molten glass G in the melting vessel 35 is dueto the fact that the flow rate of the molten glass is controlled by thescrew pumps 31, 39 so that a pressure to the molten glass is changed.

Accordingly, the weight W [ton] of the molten glass flowing in thevacuum chamber corresponds to the weight of the molten glass existingfrom the surface of the molten glass G in the melting vessel 35 to alevel which is lower by Z₂ with respect to the surface of the moltenglass G (the weight of the molten glass in a roughly hatched portion inFIG. 3).

In the present invention, various improvements and modifications can bemade within the scope of the present invention.

As described above, according to the present invention, in conductingthe degassing of molten glass flow wherein molten glass is introducedinto a vacuum chamber which can provide a pressure range of higher than38 [mmHg] but lower than (P₀-50) [mmHg] with respect to a pressure Papplied to the molten glass in a melting vessel so as to perform thedegassing of the molten glass, and the molten glass after degassing isdischarged at a flow rate of Q [ton/hr] under an atmosphere of pressureP [mmHg], a staying time of the molten glass in the vacuum chamberobtained by dividing the weight W of the molten glass flowing in thevacuum chamber by the flow rate Q [ton/hr] of the molten glass isdetermined to be in a range of 0.12-4.8 hours, whereby molten glass freefrom entrainment of bubbles can effectively and certainly be obtained.

Further, the molten glass without any bubbles can effectively andcertainly be obtained by determining the depth of the molten glass, thesurface area of the molten glass surface in the vacuum degassing vesseland the cross-sectional area of the flow path in the downfalling pipe tobe within predetermined ranges.

What is claimed is:
 1. A vacuum degassing method for molten glasscomprising: feeding, under an atmosphere of pressure P in mmHg, moltenglass into a vacuum chamber configured to render a pressure to themolten glass to be in a range of 38 mmHg-(P-50) mmHg so as to performdegassing to the molten glass, said vacuum chamber including a vacuumdegassing vessel in which the molten glass is passed in a substantiallyhorizontal state and is degassed, an uprising pipe through which themolten glass is introduced to the vacuum degassing vessel, and adownfalling pipe connected to the vacuum degassing vessel; anddischarging, via the downfalling pipe, the degassed molten glass fromthe vacuum degassing vessel at a flow rate of Q ton/hr under theatmosphere of pressure P mmHg, wherein a staying time of the moltenglass in the vacuum chamber is in a range of 0.12-0.823 hours, which isobtained by dividing a weight W in tons of the molten glass in theuprising pipe, the downfalling pipe and the vacuum degassing vesselincluded in the vacuum chamber by a flow rate Q ton/hr of the moltenglass.
 2. The method according to claim 1, wherein a viscosity of themolten glass is in a range between 500 to 5000 poises.
 3. The methodaccording to claim 1, wherein a melting vessel is connected to theuprising pipe and stores the molten glass prior to the molten glassbeing introduced into the degassing vessel via the uprising pipe, andwherein a surface level of the molten glass in the melting vessel ismade to always substantially coincide with a surface level of the moltenglass in the degassing vessel.
 4. The method according to claim 3,wherein a downstream pit is connected to the downfalling pipe andreceives the discharged degassed molten glass, and wherein the meltingvessel, the uprising pipe, the downfalling pipe and the downstream pitare formed as a single unit.
 5. The method according to claim 1, whereina depth of the molten glass in the degassing vessel is about half asmuch as a height of the degassing vessel.